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Turkey on Sunday officially recognised Russia's attacks on Ukraine as a "state of war" and said it was implementing an international treaty giving it the power to limit warships' passage through the strategic Dardanelles and Bosphorus straits.
"It's no longer a military operation but a state of war," Foreign Minister Mevlut Cavusoglu said.
"In this case, of course, whichever of them, it can be Russia or Ukraine, we apply the Montreux agreement."
The 1936 Montreux Convention governs the free movement of commercial ships in peacetime through the Bosphorus and the Dardanelles straits and grants Turkey the right to block warships in wartime if threatened.